FIG. 1 shows a prior art flexible polyethylene corrugated pipe 10 used in an irrigation system. The pipe 10 is centered on an axis 13 and extends axially from a front open end 15 to a rear open end 17. A bell-shaped section 20, or “bell,” of the pipe 10 is located at the rear end 17. The pipe 10 also includes annular corrugations 30 arranged in a linear series extending axially from the bell 20 to the front end 15. A spigot 40 of the pipe 10 comprises the front-most corrugation 42 and an annular rubber gasket 44. The spigot 40 can be inserted into the bell of another pipe (not shown) to join the pipes together with a sealed joint.
As shown in FIG. 2A, each corrugation 30 has a cylindrical inner wall 50 defining an inner cavity 51 centered on the axis 13. Additionally, each corrugation has an outer wall 60 adjoining the inner wall 50 to define a closed annular outer cavity 61 centered on the axis 13.
During use, as illustrated in FIG. 2A, water 66 is conducted axially through the inner cavity 51. A blockage can occur downstream of the pipe 10 due to, for example, a downstream valve being shut off. As the water 66 continues to flow from upstream, while being blocked from exiting downstream, the water level in the inner cavity 51 rises, as illustrated in FIG. 2B. The entire column of water flowing toward the pipe 10 has a speed and thus an inertia. The inertia can be significant, because it equals the speed of the water times the mass of the entire column of water flowing toward the pipe 10.
At some point, the water 66 can entirely fill the inner cavity 51, as illustrated in FIG. 2C. At that moment, with suddenly no more space to contain further incoming water, the inertia is suddenly dissipated by the impact of the water 66 against the inner wall 50 of the pipe 10. The impact, called water hammer, is manifested as a peak pressure within the inner cavity 51.
The pipe 10 must be designed to withstand the peak pressure, so that the pipe 10 will not rupture and the joint will not leak. This requires making the pipe walls thicker than would be necessary if the peak pressure were lower, and thus increases cost and weight of the pipe 10.